Is this what what your looking for.Amplifier Power Required
A "rule of thumb" that I've seen in high-efficiency speaker circles is for the speaker/amp combination to be able to reach, on paper at least, 102 dB at one meter.
Each doubling of power is theoretically 3 dB more output, so 8 watts is three doublings of the 1 watt efficiency = 9 dB more output than the speaker's rated efficiency. So in theory if the speaker is rated a 93 dB for a 1 watt input, you'll be able to reach 102 dB with an 8 watt amp.
Now there are other considerations. For example, will the speaker run into problems before you get to 102 dB? Many single-driver speakers can handle much more than 8 watts thermally, but in the bass region will be driven beyond their linear excursion limits with much less than 8 watts input and therefore will lose coherence and articulation as the voice coil spends time outside of the magnetic gap.
Also, what does the speaker's impedance curve look like? If the impedance curve has nasty swings (especially above the bass impedance peaks), then the amplifier may clip long before it gets to 8 watts.
In defiance of the 102 dB rule of thumb, I paired up a smooth-impedance 92 dB speaker with a 5-watt amp at an audio show, and the only audible distortion came on loud solo piano. So that would have been enough power for some people, but not enough for others.
Best of luck,
Zippyy is correct on both counts.
SPL falls off by 6 dB for every doubling of distance from a point source under anechoic conditions, but then in a non-anechoic room you get back energy from the reverberant field - and how much of a difference that makes depends on the room acoustics and distance from the speakers.
Once all is factored in, actual SPL at a listening distance of 10 feet in a semi-damped/semi-reverberant room is often about 3 dB less than the calculated 1 meter peak SPL for a single speaker. In other words, the 102 dB rule of thumb described above typically translates to about 99 dB peak at the listening position (adequate for some people, but not for everyone). But, it's much less math-intensive to do the 1 meter calculation for a single speaker. Obviously if you know your room is on the overdamped side and/or you listen from fairly far away in a big room and/or you like it fairly loud, you'll need to shoot for an amplifier/speaker combination that can deliver higher sound pressure levels.
To cover my bases, I should point out that the 102 dB rule of thumb applies to tube amplifiers only. Solid state amplifiers produce audible distortion at lower average power output levels because their characteristics when driven into clipping are more objectionable - so if you're using a solid state amp, use a 105 dB rule of thumb. Also, this only holds for fairly high efficency speakers - if you're doing this calculation with 85 dB speakers and a big amplifier, you're probably going to run into some thermal compression so factor in another 2-3 dB or so. In other words, 8 tube watts on a 93 dB tube-friendly speaker will probably produce audible signs of distress at around the same average (not peak) sound pressure level as 150-200 solid state watts on an 85 dB speaker.
Makes the case for hi-eff horns doesnt it;) The -6db for every doubling of distance is not fully true for large horns. I have a pair that has greater SPL at 15ft than at 7ft. Large horns like these dont lose SPL to distance as much as non horn loudspeakers. Why horns rule in PA, sound reinforcement,theaters and yes hi-quality home audio reproduction. Every year more manufacters use horns or wave guides;) Must be reason...If I was going with a 8 watt tube amp depending on my tastes budget room etc. I would look for loudspeakers with min 90db 8 ohms -114db 8ohms.
Don't forget that the crossover used can affect how much power you will need, a complex crossover with lots of parts will need more power to sound dynamic than a simple or no crossover will. Some speakers have only a cap on the tweeters while others use no crossover(single driver designs) Tell us which speakers you are considering or what is your price range and I'm sure you will get lots of suggestions. For low cost try Hornshop Horns, Reference3 A Mm De Capo's will work with 8 watts also (92db@1watt1meter and only a cap on the tweets. I use Sonist Concerto2 (95db rated with good results paired with 5 watt amp.
Groundhog- Read this before you get too confused between amplifier output power, SPL and perceived loudness levels: (http://www.audioholics.com/education/frequently-asked-questions/relationship-between-watts-and-dbs) You'll notice that it actually takes 10 times the amplifier power to produce an SPL that SOUNDS twice as loud to our ears. That is an important factor to consider with your goals.